Conventionally, for purpose of improvement of catalytic conversion efficiency of a catalyst used for purification of exhaust gas, an emission control system for an internal combustion engine includes exhaust-gas sensors (e.g., an air/fuel sensor and an oxygen sensor) that are respectively disposed upstream and downstream of the catalyst in a flow direction of the exhaust gas. The exhaust-gas sensors detect an air-fuel ratio of the exhaust gas or detects whether the exhaust gas is rich or lean. The emission control system performs a “main feedback control” and a “sub feedback control”. In the main feedback control, the emission control system corrects a fuel injection amount based on an output of the upstream exhaust-gas sensor so that an air-fuel ratio of the exhaust gas flowing upstream of the catalyst becomes equal to an upstream target air-fuel ratio. In the sub feedback control, the emission control system corrects the upstream target air-fuel ratio, a correction amount used in the main feedback control, or the fuel injection amount based on an output of the downstream exhaust-gas sensor so that an air-fuel ratio of the exhaust gas flowing downstream of the catalyst becomes equal to a downstream target air-fuel ratio.
When the air-fuel ratio of the exhaust gas changes from rich to lean or from lean to rich, an output change of the exhaust-gas sensor, such as an oxygen sensor, may lag behind a change of an actual air-fuel ratio of the exhaust gas. Thus, the exhaust-gas sensor may have a room for improvement in its detection responsiveness.
For example, as described in Patent Document 1 (JP 8-20414 B2 corresponding to U.S. Pat. No. 4,741,817 A), at least one of an auxiliary electrochemical cell is incorporated into an inside of a gas sensor for increase in its detection responsiveness.
In order to let the air-fuel ratio of exhaust gas flowing downstream of the catalyst converge smoothly on the downstream target air-fuel ratio in a main feedback control and a sub feedback control, emission control systems are disclosed in Patent Document 2 (JP 2518247 B2) and Patent Document 3 (JP 3826996 B2). In Patent Document 2, an updating amount of a constant used in a feedback control is increased in accordance with increase of a difference between an output of an exhaust-gas sensor located downstream of a catalyst and a predetermined value corresponding to the stoichiometric air-fuel ratio. Additionally, a correction amount of an air-fuel ratio of exhaust gas flowing downstream of the catalyst is calculated depending on an output of an exhaust-gas sensor located upstream of the catalyst and the constant used in the feedback control. In Patent Document 3, a middle target value is set between an air-fuel ratio detected by an exhaust-gas sensor located downstream of a catalyst and a target air-fuel ratio downstream of the catalyst. A correction amount of an upstream target air-fuel ratio is calculated based on the air fuel-ratio detected by the downstream exhaust-gas sensor and the middle target value.
In Patent Document 1, the auxiliary electrochemical cell is necessarily incorporated into the inside of the gas sensor. Thus, when the auxiliary electrochemical cell is incorporated into a general gas sensor that does not have an auxiliary electrochemical cell, the general gas sensor may need to be changed greatly in structure. For practical use, the gas sensor may be required to be changed in design, and a manufacturing cost of the gas sensor may be increased.
In the emission control systems described in Patent Documents 2 and 3, correction of the air-fuel ratio of exhaust gas flowing downstream of the catalyst in the sub feedback control is not switched until the downstream exhaust-gas sensor detects that a conversion efficiency of the catalyst with respect to NOx or HC becomes low (i.e., emission rate of NOx or HC becomes high). Thus, it may be difficult to keep a conversion efficiency of the catalyst high (within a purification window). Therefore, an emission rate of a harmful material such as NOx and HC may increase.